Cash-strapped motorists will be hit with another rise in petrol prices - expected to go up by another 4p a litre within days, industry sources warned today.

Forecourt retailers are expected to pass on increased wholesale costs which they are blaming on speculators driving up the price of fuel.

Petrol prices have been going up after more than than three months of falling costs at forecourts and the AA has accused retailers of failing to fully pass on a 2p fall in diesel wholesale costs in November and December that, with VAT, were worth 2.5p at the pump.

The 4p rise would add £2 to cost of filling a Vauxhall Astra.

The Petrol Retailers Association said the hike in petrol costs was due to an unexplained leap in wholesale prices of 5p a litre since Christmas and warned that wholesale cost changes could see them go up by another 4p in coming days.

Next week the Office of Fair Trading will decide if it will launch an investigation into the fuel market and whether reductions in the price of crude oil are being passed on to motorists.

The UK retail road fuels sector is estimated to be worth around £32bn, according to the OFT.

Brian Madderson, chairman of the PRA, said: “We cannot explain to our customers why the wholesale price is going up so much - it is not due to Government tax, it is not due to Brent crude going up and it is not due to the weak currency exchange.

“So are traders, bankers or speculators taking British motorists for a ride?

“Motorists will understand that, in this winter weather, demand for petrol in this country has been at a low ebb.

“If there is no massive demand, why have wholesale costs been ramped up by nearly 5p a litre?

“We do not understand it - yet once again we are going to be accused of profiteering at the pumps when that is simply not true.”

He accepted that retailers had failed to pass on the 2p fall in diesel wholesale costs ahead of Christmas, but said that was due to the “horrendous time” they had during the summer.

According to the latest figures released by Experian Catalist, petrol is selling on average for 132.83 pence a litre and diesel at 140.4 pence a litre.

Edmund King, president of the AA, said: “Another new year, another new round of pump price rises after the industry failed to pass on fully wholesale price savings.

“The insight we are now getting on wholesale price movements rams home the need for this information to be out in the public domain immediately.

“Wholesale petrol prices turned upward in the first week of January, average pump prices six days later.

"If falls in wholesale were reflected as quickly, no one would mind - but they’re not.”

In its latest price report, the AA said a rise of 0.75p since the new year left average prices at 132.71p a litre.

Diesel is marginally cheaper than a month ago, falling by a third of a penny in the third week of December and remaining around 140p per litre throughout Christmas and early January.

But the average of 140.32p a litre, compared to 140.38p in mid-December means a litre of diesel has been on average 8p a litre more expensive than petrol, compared with a 5p difference in the summer, something the AA blames on the drop in wholesale costs not being passed on.

It claimed that in December into this month, the fuel industry failed to pass on fully a 2p fall in diesel wholesale.

Between late November and early December, it said, the cost of diesel to retailers fell from around 53p a litre to below 51p, but the average pump price only fell from 141.5p to 140p a litre.

Since the start of the new year, the pound has weakened against the dollar in international trading, putting pressure on prices to move upwards.

Average petrol prices are unchanged in Wales but up 0.7p a litre in the West Midlands and East Anglia.

Average diesel prices show little change month on month, apart from Scotland where average prices are down 0.7p a litre.

Yorkshire and Humberside remains the cheapest area in the UK for petrol and diesel at 132.1p and 139.7p a litre respectively, with Northern Ireland the dearest at 133.6p and 141.0p per litre.

RAC technical director David Bizley said: “It’s hugely disappointing for motorists that another painful price rise is looming, particularly as it comes so quickly after the good news that the Government has scrapped the planned January 3p fuel duty increase.

“It would, however, be extremely ironic if a price rise was to hit in the very week the Office of Fair Trading plans to publish the findings of its review of UK fuel prices.

“Rising fuel prices cause economic hardship for millions and hinder the growth of the economy, and for the tens of millions of households who rely on their car, this news could not have come at a worse time.”